In the metal casting process, molten metal is passed into a mold cavity. For some types of casting, mold cavities with false, or moving, bottoms are used. As the molten metal enters the mold cavity, generally from the top, the false bottom lowers at a rate related to the rate of flow of the molten metal. The molten metal that has solidified near the sides can be used to retain the liquid and partially liquid metal in the molten sump. Metal can be 99.9% solid (e.g., fully solid), 100% liquid, and anywhere in between. The molten sump can take on a V-shape, U-shape, or W-shape, due to the increasing thickness of the solid regions as the molten metal cools. The interface between the solid and liquid metal is sometimes referred to as the solidifying interface.
As the molten metal in the molten sump becomes between approximately 0% solid to approximately 5% solid, nucleation can occur and small crystals of the metal can form. These small (e.g., nanometer size) crystals begin to form as nuclei, which continue to grow in preferential directions to form dendrites as the molten metal cools. As the molten metal cools to the dendrite coherency point (e.g., 632° C. in 5182 aluminum used for beverage can ends), the dendrites begin to stick together. Depending on the temperature and percent solids of the molten metal, crystals can include or trap different particles (e.g., intermetallics or hydrogen bubbles), such as particles of FeAl6, Mg2Si, FeAl3, Al8Mg5, and gross H2, in certain alloys of aluminum.
Additionally, when crystals near the edge of the molten sump contract during cooling, yet-to-solidify liquid compositions or particles can be rejected or squeezed out of the crystals (e.g., out from between the dendrites of the crystals) and can accumulate in the molten sump, resulting in an uneven balance of particles or less soluble alloying elements within the ingot. These particles can move independently of the solidifying interface and have a variety of densities and buoyant responses, resulting in preferential settling within the solidifying ingot. Additionally, there can be stagnation regions within the sump.
The inhomogenous distribution of alloying elements on the length scale of a grain is known as microsegregation. In contrast, macrosegregation is the chemical inhomogeneity over a length scale larger than a grain (or number of grains), such as up to the length scale of meters.
Macrosegregation can result in poor material properties, which may be particularly undesirable for certain uses, such as aerospace frames. Unlike microsegregation, macrosegregation cannot be fixed through typical homogenization practices (i.e., prior to hot rolling). While some macrosegregation intermetallics may be broken up during rolling (e.g., FeAl6, FeAlSi), some intermetallics take on shapes that are resistant to being broken up during rolling (e.g., FeAl3).
While the addition of new, hot liquid metal into the metal sump creates some mixing, additional mixing can be desired. Some current mixing approaches in the public domain do not work well as they increase oxide generation.
Further, successful mixing of aluminum includes challenges not present in other metals. Contact mixing of aluminum can result in the formation of structure-weakening oxides and inclusions that result in an undesirable cast product. Non-contact mixing of aluminum can be difficult due to the thermal, magnetic, and electrical conductivity characteristics of the aluminum.
In addition to oxide formation through some mixing approaches, metal oxides can form and collect as the molten metal cascades into the mold cavity. Metal oxides, hydrogen, and/or other inclusions can collect as a froth or oxide slag on the top of the molten metal within the mold cavity. For example, during aluminum casting, some examples of metal oxides include aluminum oxide, aluminum manganese oxide, and aluminum magnesium oxide.
In direct chill casting, water or other coolant is used to cool the molten metal as it solidifies into an ingot as the false bottom of the mold cavity lowers. Metal oxides do not diffuse heat as well as the pure metal. Metal oxides that reach the side surfaces of the forming ingot (e.g., through “rollover” where the metal oxide from the upper surface of the molten metal migrates over the meniscus between the upper surface and a side surface) may contact the coolant and create a heat transfer barrier at that surface. In turn, areas with metal oxide contract at a different rate than the remainder of the metal, which can cause stress points and thus fractures or failures in the resultant ingot or other cast metal. Even small defects in a piece of cast metal can result in much larger defects when the cast metal is rolled if not adequately scalped to remove any artifact of an earlier oxide patch.
Control of metal oxide rollover can be partially achieved through the use of skimmers. Skimmers, however, do not fully control metal oxide rollover and can add moisture to the casting process. Additionally, skimmers are not typically used when casting certain alloys, such as aluminum-magnesium alloys. Skimmers can form unwanted inclusions in the metal melt. Manual oxide removal by an operator is extremely dangerous and time-consuming and risks introducing other oxides into the metal. Thus, it can be desirable to control metal oxide migration during the casting process.